The present invention relates to a steel used in the separators of solid-oxide type fuel cells.
Because of their excellent features, such as high power generation efficiency, small emissions of SOx, NOx and CO2, good response to load variations and compact size, fuel cells are expected to be used in a wide range of applications in power generation systems such as a large-scale centralized generation type, a decentralized generation type provided near cities, and an independent power plant type, as a substitute for thermal power generation.
Types of fuel cells are sorted according to the used electrolyte into the phosphoric acid type, the fused carbonate type, the solid-oxide type and the solid-polymer type. Among others, the solid-oxide type fuel cells use as the electrolyte thereof ceramics such as stabilized zirconia and have been operated at high temperatures near 1000xc2x0 C.
The above solid-oxide type fuel cell is regarded to be very promising as the next-generation power supply source because it has excellent features as explained below. That is, because the solid-oxide type fuel cell is operated at high temperatures, it is unnecessary to use a catalyst for electrode reactions, the internal modification of fossil fuels by high temperatures being possible and various kinds of fuels such as coal gas being able to be used, the high-efficiency power generation being possible by a so-called combined-cycle power generation in which a combination with a gas turbine, steam turbine, etc. is adopted by utilizing high-temperature waste heat, and the solid-oxide fuel cell is compact because all components are solids.
However, there are still many problems to be examined in the practical application of the solid-oxide type fuel cell, and particularly in the case of a planar type fuel cell that permits a high output density, a separator is present as an important component.
This separator supports the three layers of electrolyte, anode and cathode, define gas passages, and at the same time causes electric currents to flow. Therefore, the separator is required to provide properties such as electrical conductivity at high temperatures, oxidation resistance, and a small difference in thermal expansion from the electrolyte and hence in consideration of such required properties, electrically conductive ceramics have been frequently used. However, because ceramics have inferior machinability and are expensive, there are problems in terms of large size design and practical application of fuel cells.
For this reason, the development of a separator made of a metallic material that is inexpensive and reliable is demanded. When a usual metallic material is used at 1000xc2x0 C., the surface of the metallic material is oxidized and an oxide film is formed. However, when a metallic material is used as the material for the separator, it is necessary that this oxide film be stable so that oxidization does not proceed, and at the same time, it is necessary that the oxide film have an electrical conductivity.
In order to meet such required properties, in JP-A-6-264193 is proposed as the metallic material for solid-oxide type fuel cells an austenitic stainless steel that consists of not more than 0.1% C, 0.5 to 3.0% Si, not more than 3.0% Mn, 15 to 30% Cr, 20 to 60% Ni, 2.5 to 5.5% Al, and the balance of Fe.
Also, in JP-A-7-166301 is proposed as a separator of solid-electrolyte type fuel cells an alloy containing 60 to 82% Fe, 18 to 40% Cr, and additive elements that reduce the contact resistance between the above-described single cell and the cathode (La, Y, Ce or Al being singly added). Furthermore, in JP-A-7-145454 is proposed as a metallic material for solid-electrolyte type fuel cells a material that comprises 5 to 30% Cr, 3 to 45% Co, not more than 1% La, and the balance of Fe.
Recently, however, dramatic progress has been made in the improvement of solid-electrolyte type fuel cells and it has become possible to lower operating temperature from conventional levels from about 1000xc2x0 C. to the range of 700 to 950xc2x0 C. or so. Therefore, it is expected that practical application will be accelerated.
Because the material disclosed in JP-A-6-264193 contains considerable amounts of Al and Cr, the oxide film on the surface comprises Al-base oxides as the main composition and further contains Cr-base oxides. However, as is described below, because Al-base oxides have a low electrical conductivity, this material is not always sufficient for use in separators of solid-oxide type fuel cell, and besides because austenitic stainless steels have a larger coefficient of thermal expansion than the stabilized zirconia of electrolyte, they are apt to cause a deterioration of the performance of cells caused by the crack formation etc. of the electrolyte due to the heat cycles associated with the start and stop of cells, thus posing a problem in stability in the case of long time of use. In addition, because expensive Ni is contained in large amounts, the price is high and this is not sufficient for the practical application of fuel cells.
In contrast to this, the materials disclosed in JP-A-7-166301 and JP-A-7-145454 have lower values of coefficient of thermal expansion than the austenitic stainless steels and these values of coefficient of thermal expansion are close to that of the stabilized zirconia of electrolyte. Therefore, these materials are favorable in terms of stability in the case of long time of use and besides they have good electrical conductivity. However, the oxidation resistance after long time of use is insufficient, and particularly these materials promote the phenomenon of exfoliation associated with an increase in oxide films with the result that the grooves provided in the separator as gas passages in the cell are narrowed, thus posing the problem of a deterioration of the cell function.
Also, the materials disclosed in JP-A-8-35042 and JP-A-8-277441 have lower values of coefficient of thermal expansion than austenitic stainless steels, and these values are close to that of the stabilized zirconia of the electrolyte. Therefore, these materials are favorable in terms of stability in the case of long time of use. However, electrical conductivity, which is important as a property of a separator material, is not taken into consideration at all.
Also, the materials disclosed in JP-A-9-157801 and JP-A-10-280103 have low values of coefficient of thermal expansion up to 1000xc2x0 C. and these values are close to that of the stabilized zirconia of electrolyte. Therefore, these materials are favorable in terms of stability in the case of long time of use and besides they have also good oxidation resistance at 1000xc2x0 C. and have a good electrical conductivity.
However, the known materials described above were all developed to obtain good properties as separators of solid-oxide type fuel cells that operate at 1000xc2x0 C. and the properties at the operating temperatures of recent solid-oxide type fuel cells of 700 to 950xc2x0 C. or so are not taken into consideration at all.
Because the solid-oxide type fuel cells repeat operations and stops, stresses due to heat cycles act on the separator, which is a cell component. Because there is a fear of a failure of the separator due to thermal stresses especially in the case of low impact properties at room temperature, it is necessary for a steel for separators to have impact properties high enough to withstand thermal stresses. In the known materials, however, impact properties at room temperature are not taken into consideration at all.
The object of the invention is to provide a steel for separators of solid-oxide type fuel cells, which steel makes oxide films having good electrical conductivity at 700 to 950xc2x0 C. or so, which steel has good oxidation resistance and, in particular, the resistance to exfoliation even in the case of long time of use, which steel is excellent in impact properties at room temperature, which steel is small in the difference of thermal expansion between the electrolyte and the steel, and which steel is inexpensive.
As a result of various examinations, the present inventors decided to use ferritic metallic materials. The first reason for this is as follows. That is, in view of the fact that the mean coefficient of thermal expansion of stabilized zirconia, which is the electrolyte of solid oxides, at room temperature to about 750xc2x0 C. is about 11xc3x9710xe2x88x926/xc2x0 C., whereas that of ordinary austenitic metallic materials is about 16xc3x9710xe2x88x926/xc2x0 C., there being a great difference in thermal expansion between the two, the inventors have thought that there is a problem in stability in the case of long time of use.
The second reason is that in general, austenitic metallic materials contain expensive Ni and hence are expensive, whereas ferritic metallic materials comprise Fe as the base composition and do not contain Ni or contain only a small amount of Ni and, therefore, they are inexpensive.
Next, the inventors conducted various examinations as to the electrical conductivity of oxide films formed.
Oxides of Al and oxides of Cr are known as representative oxide films that have protective properties. At high temperatures of 700 to 950xc2x0 C. or so, Al2O3 has generally a greater protective action and is favorable. However, when the electric resistance of materials on which Al2O3 films are formed was measured, the electric resistance value was very large and it became apparent that the materials on which Al2O3 films are formed cannot be used as separators.
On the other hand, it was found that materials on which Cr2O3 films are formed have sufficiently small electric resistance and hence can be used in separators. In the invention, therefore, it was decided to use ferritic metallic materials that make on their surfaces thereof oxide films comprising as the main component thereof Cr-base oxides, i.e., Fe-Cr-base materials.
Next, the oxidation resistance that poses a problem in the case of long time of use is explained. As explained above, at 700 to 950xc2x0 C. or so, usually the oxidation resistance of the Cr-base oxide films is inferior to that of Al-base oxide films. Also, in the case where Cr-base oxide films are the main component, an Fe-base alloy (for example, Fe-Cr alloys represented by JIS-SUS 430) is inferior in oxidation resistance to an Ni-base alloy (for example, Ni-Cr alloys represented by JIS-NCF600).
Therefore, if an Fe-Cr-base material is used as the base composition following the above-described policy, it becomes very difficult to meet oxidation resistance.
The inventors have conducted various examinations to solve this problem and have found that, by adding to an Fe-Cr-base material one or more elements selected from the group consisting of Y, REM (Rare earth metal) and Zr, by suppressing the Al content to a low level, and further by adding Si and Mn by small amounts, it becomes possible to obtain a good oxidation resistance and, in particular, a good resistance to exfoliation although the steel has the Cr-base oxides as the main component of the films, so that the state of oxide films becomes stable even after long time of heating.
In particular, when Zr is added in combination to Y and/or rare-earth metals, the resistance to the exfoliation is most improved. Also, the inventors have found that even when each of the above-described elements is added, oxide films are mainly Cr-base oxide films and hence the electric resistance does not increase so much.
Furthermore, Y, rare-earth metals and Zr, which are effective in obtaining good oxidation resistance, are apt to become inclusions by the occurrence of sulfides and oxides when an alloy contains much amounts of S and O (oxygen). If Y, rare-earth metals and Zr become inclusions and are fixed, the amounts of Y, rare-earth metals and Zr that exist in a solid solution state in the matrix decrease, and their effective amounts decrease which are capable of contributing to the suppression of the growth of an oxide film, densification thereof and improvement of the adhesion thereof. Therefore, in order to ensure that added Y, rare-earth metals and Zr act effectively, it is effective to minimize the inclusions of these elements. Thus, the inventors have found that lowering the contents of impurities such as S and O (oxygen) is necessary for keeping the good oxidation resistance.
Furthermore, the inventors have found that, because N and B have also the fear of causing compounds by combining with a part of the elements such as, for example, La effective for keeping the oxidation resistance, it is effective to reduce these elements present as impurities to low amounts. Also, because the inventors have found that B in particular impairs the smoothness of the oxide film surface and reduces the contact resistance, it is necessary to suppress to a low level the amount of B contained in the steel for separators from the standpoint of the contact resistance.
Also, because Ti is an element that reduces oxidation resistance, it is also necessary to suppress to a low level the amount of Ti present as an impurity.
In working a steel as the material for separators to fabricate separators, it is necessary to reduce the hardness of the steel, and for this purpose it is necessary to perform appropriate annealing. This annealing is effective not only in hardness adjustment, but also in the adjustment of the ferrite grain size, and by ensuring an appropriate fine structure it is possible to improve also the impact properties.
As described above, the inventors have achieved the invention by optimizing heat treatment conditions, structure and mechanical properties while performing the adjustment of alloy components.
That is, in the first aspect of the invention, there is provided a steel for the separators of solid-oxide type fuel cells, which steel consists essentially, by mass, of not more than 0.2% C; not more than 1.0% Si exclusive of zero; not more than 1.0% Mn exclusive of zero; not more than 2% Ni; 15 to 30% Cr; not more than 1% Al; at least one kind selected from the group consisting of not more than 0.5% Y, not more than 0.2% rare earth elements (REM) and not more than 1% Zr; and the balance being Fe and incidental impurities including not more than 0.015% S, not more than 0.010% O (oxygen), not more than 0.050% N, and not more than 0.0030% B, the contents of the elements satisfying the formula (1) defined by
(O (oxygen)+2S)/(0.27Y+0.035Zr+0.16REM)xe2x89xa62.0xe2x80x83xe2x80x83(1), 
the steel having a hardness not more than 280 HV and fine grains of an average ferrite grain size number not less than ASTM No.2. In this formula (1), each of the atomic symbols means the amount of the element contained in the steel.
Further, in the second aspect of the invention, there is provided a steel for separators of solid-oxide type fuel cells, which steel consists essentially, by mass, of not more than 0.1% C; not more than 1.0% Si exclusive of zero; not more than 1.0% Mn exclusive of zero; not more than 1% Ni; 17 to 26% Cr; not less than 0.001 to less than 0.5% Al; 0.01 to 0.8% Zr; at least one kind selected from the group consisting of 0.01 to 0.3% Y, and 0.005 to 0.1% rare earth elements (REM); and the balance being Fe and incidental impurities including not more than 0.015% S, not more than 0.010% O (oxygen), not more than 0.020% N, and not more than 0.0030% B, the contents of the elements satisfying the formula (1) defined by
(O (oxygen)+2S)/(0.27Y+0.035Zr+0.16REM)xe2x89xa62.0xe2x80x83xe2x80x83(1), 
the steel having a hardness not more than 280 HV and fine grains of an average ferrite grain size number not less than ASTM No.2.
In the third aspect of the invention, there is provided a steel for separators of solid-oxide type fuel cells, which steel consists essentially, by mass, of not more than 0.1% C; less than 0.2% Si exclusive of zero; not more than 1.0% Mn exclusive of zero; not more than 1% Ni; 17 to 26% Cr; not less than 0.001 to less than 0.5% Al; 0.01 to 0.8% Zr; at least one kind selected from the group consisting of 0.01 to 0.3% Y, and 0.005 to 0.1% rare earth elements (REM); and the balance being Fe and incidental impurities including not more than 0.015% S, not more than 0.010% O (oxygen), not more than 0.020% N, and not more than 0.0030% B, the contents of the elements satisfying the formula (1) defined by
(O (oxygen)+2S)/(0.27Y+0.035Zr+0.16REM)xe2x89xa62.0xe2x80x83xe2x80x83(1), 
the steel having a hardness not more than 280 HV and fine grains of an average ferrite grain size number not less than ASTM No.2.
In the fourth aspect of the invention, there is provided a steel for separators of solid-oxide type fuel cells, which steel consists essentially, by mass, of not more than 0.1% C; less than 0.2% Si exclusive of zero; less than 0.2% Mn exclusive of zero; not more than 1% Ni; 17 to 26% Cr; not less than 0.001 to less than 0.5% Al; 0.01 to 0.8% Zr; at least one kind selected from the group consisting of 0.01 to 0.3% Y, and 0.005 to 0.1% rare earth elements (REM); and the balance being Fe and incidental impurities including not more than 0.015% S, not more than 0.010% O (oxygen), not more than 0.020% N, and not more than 0.0030% B, the contents of the elements satisfying the formula (1) defined by
(O (oxygen)+2S)/(0.27Y+0.035Zr+0.16REM)xe2x89xa62.0xe2x80x83xe2x80x83(1), 
the steel having a hardness not more than 280 HV and fine grains of an average ferrite grain size number not less than ASTM No.2.
In the fifth aspect of the invention, there is provided a steel for separators of solid-oxide type fuel cells, which steel consists essentially, by mass, of not more than 0.08% C; not more than 0.6% Si exclusive of zero; not more than 0.5% Mn exclusive of zero; not more than 0.5% Ni; 18 to 25% Cr; not less than 0.001 to less than 0.5% Al; 0.005 to 0.1% La, 0.01 to 0.6% Zr; and the balance being Fe and incidental impurities including not more than 0.1% Ti, not more than 0.008% S, not more than 0.008% O (oxygen), not more than 0.020% N, and not more than 0.0020 B, the contents of the elements satisfying the formula (2) defined by
(O (oxygen)+2S)/(0.035Zr+0.16La)xe2x89xa62.0xe2x80x83xe2x80x83(2), 
the steel having a hardness not more than 280 HV and fine grains of an average ferrite grain size number not less than ASTM No.2.
In a steel for separators of solid-oxide type fuel cells according to the invention, preferably, the B content is restricted to less than 0.0010%, the average ferrite grain size number is not less than ASTM No. 3, which provides fine grains, and the 2-mm V-notch Charpy impact value at 20xc2x0 C. is not less than 10 J/cm2.
Preferably, there is provided a steel for separators of solid-oxide type fuel cells, which steel further contains Mo alone or two kinds of Mo and W by an amount not more than 5.0% in terms of (Mo+xc2xdW).
More preferably, a steel for separators of solid-oxide type fuel cells according to the invention further contains at least one kind of 0.01 to 1.0 in total selected from the group consisting of V, Nb, Ta, and Hf.
More preferably, a steel for separators of solid-oxide type fuel cells according to the invention has a 2 mm-V notch Charpy impact value not less than 8 J/cm2.
In a steel for separators of solid-oxide type fuel cells according to invention, more preferably, the electric resistance of an oxide film at 750xc2x0 C. after heating at 750xc2x0 C. for 1000 hours is not more than 100 mxcexa9xc2x7cm2 and no exfoliation of surface oxide scale substantially occurs after further heating at 850xc2x0 C. for 100 hours.
In a steel for separators of solid-oxide type fuel cells according to invention, also more preferably, the electric resistance of an oxide film at 750xc2x0 C. after heating at 750xc2x0 C. for 1000 hours is not more than 50 mxcexa9xc2x7cm2 and no exfoliation of surface oxide scale substantially occurs after further heating at 850xc2x0 C. for 100 hours.
The reason for the restriction of the components of the invention is explained below.
Although C has the function of increasing the high-temperature strength by making carbides, this element deteriorates the workability and reduces the amount of Cr, which is effective for the oxidation resistance, by combining with Cr. Accordingly, the C content is restricted to be not more than 0.2%. The C content is preferably not more than 0.1% and more preferably not more than 0.08%.
Si participates in the making of the film, the main component of which is a Cr-base oxide layer, on the inner surface of the groove of a high-temperature passage provided in a separator, and even in the case of long time of use this element has the function of preventing the oxide film from growing in excess of necessity and from undergoing the exfoliation phenomenon.
It is thought that, as one of the effects of Si, this element improves the oxidation resistance by making a thin and discontinuous SiO2 film probably near the interface defined between a Cr2O3 oxide film and the base metal.
Further, the above SiO2 film produces, at the interface between the base metal and the Cr2O3 film, a state in which the base metal, Cr2O3 film and SiO2 film are finely intertwined with each other, thereby increasing the adhesion to the base metal and being effective in preventing the exfoliation of the Cr2O3 film.
Although this effect is especially great at high temperatures of not less than 1000xc2x0 C. and is not always great at 700 to 950xc2x0 C., it is necessary to add a small amount of Si in order to obtain the above-described effect. On the other hand, excessive addition of Si causes a deterioration of workability and toughness and makes the SiO2 film too thick and continuous, causing the exfoliation of the oxide film and causing the problem of a decrease in the electrical conductivity of the film. Accordingly, the Si content is, exclusive of zero, not more than 1.0%. A preferred Si content is, exclusive of zero, not more than 0.6%, and a more preferred Si content range is, exclusive of zero, less than 0.2%.
Mn, along with Fe and Cr, makes spinel-type oxides. A spinel-type oxide layer containing Mn is formed on the outside of a Cr2O3 oxide layer. This spinel-type oxide layer has the protective function of preventing Cr from evaporating from a steel for separators which evaporation deteriorates the ceramics electrolyte of solid-oxide type fuel cells. Furthermore, because the spinel-type oxides usually have a higher oxidation rate than Cr2O3, they have the functions of reducing contact resistance by keeping the smoothness of the oxidation film and of preventing the evaporation of Cr which evaporation is detrimental to the electrolyte, although they are unfavorable for oxidation resistance itself.
On the other hand, the excessive addition thereof deteriorates the resistance to oxidation because the spinel-type oxides containing Mn have insufficient oxidation resistance as described above. Accordingly, the Mn content is restricted to be, exclusive of zero, not more than 1.0%. When the use temperature is low to be from 700 to 950xc2x0 C. or so, the Mn content may be, exclusive of zero, not more than 0.5%. At further lower use temperatures, the Mn content may be, exclusive of zero, less than 0.2%.
In the invention, Cr is an important element for keeping the good oxidation resistance and electrical conductivity both brought about by the existence of the Cr2O3 film. For this reason, the required Cr content is 15% at least. However, the excessive addition is not so effective in the improvement of oxidation resistance. On the contrary, the excessive addition thereof causes the deterioration of workability. Accordingly, the Cr content is restricted to be 15 to 30%. A preferred Cr content is 17 to 26% and a more preferred Cr content is 18 to 25%.
Y, REM and Zr have the effect of substantially improving the oxidation resistance and the electrical conductivity of the oxide film by the addition of a small amount. In particular, the improvement of the oxidation resistance becomes great especially when the addition of these elements is combined with the addition of small amounts of Si and Mn. It is thought that this is due mainly to the improvement of the adhesion of the oxide film.
In the invention, the oxidation resistance is imparted by the Cr-base oxide film alone. Adding Y, REM or Zr singly or in combination is indispensable for improving the adhesion of this Cr-base oxide film. However, because the excessive addition deteriorates hot workability, the contents of Y, REM and/or Zr is restricted to be not more than 0.5%, not more than 0.2% and not more than 1%, respectively. Preferably, the Y content is 0.01 to 0.3%, the REM content being 0.005 to 0.10%, and the Zr content is 0.01 to 0.8%.
Furthermore, by the combined addition of one or more elements selected from the group consisting of 0.01 to 0.3% Y and 0.005 to 0.10% REM with the addition of 0.01 to 0.8% Zr, the adhesion of the oxide film is further improved and the exfoliation of the oxide film can be prevented even after long time of heating. More preferably, 0.005 to 0.10% REM and 0.01% to 0.8% Zr are added in combination. Because La is most effective in improving the adhesion of the oxide film among all of the rare-earth metals, the combined addition of 0.005 to 0.10% La and 0.01 to 0.6% Zr is most preferred.
Incidentally, similarly to V, Nb, Ta and Hf, which are explained below, Zr combines with C to thereby make carbides, improves the workability by fixing C, and also contributes to the improvement of strength.
Ni is effective in improving toughness when it is added in a small amount to the steel of the invention. However, Ni is an austenite-forming element and excessive addition of Ni forms a ferrite-austenite binary-phase structure and results in an increase in the coefficient of thermal expansion and the cost increases. Furthermore, the excessive addition of Ni deteriorates the oxidation resistance. Accordingly, the Ni content is restricted to be not more than 2%. The Ni content is preferably not more than 1% and more preferably not more than 0.5%.
Al is usually added as a deoxidizer. Although an Al2O3 film is formed when Al is added in a large amount, and the Al2O3 film increases the electrical resistance of the oxide film although it is effective for improving the oxidation resistance. Accordingly, in the case of the invention, the Al content is restricted to be not more than 1% in order to prevent the Al2O3 film from occurring. Preferably, the Al content is not less than 0.001 to less than 0.5%.
Because Mo has the particular function of increasing high-temperature strength, this element may be added when it is important to obtain a high-temperature strength. Although W has an effect similar to that of Mo, W is needed to be added, in order to ensure that W brings about the same effect as Mo, in an amount twice, by mass %, as large as the Mo content. Because the addition of a large amount of W deteriorates the hot workability, it is necessary to suppress the total amount of Mo and W by adding W and Mo in combination.
Thus, since the excessive addition thereof deteriorates not only workability but also the oxidation resistance, the content of these two elements is restricted to be not more than 5% in terms of Mo+W/2. Preferably, the content of the two elements is not more than 3%.
V, Nb, Ta and Hf combine with C to thereby make carbides and improve the workability by fixing C. Although these elements contribute to an increase in strength, they make the oxides with the exception of Hf which oxides have not so good protective properties, thereby deteriorating the oxidation resistance.
Because Hf is also effective in improving the oxidation resistance, it is the most preferred elements of all. However, the addition of Hf is selected as required because this element is expensive. Furthermore, the excessive addition of V, Nb, Ta and Hf forms primary carbides in large amounts, thereby deteriorating the workability. Accordingly, in taking the workability, strength and oxidation resistance into consideration, V, Nb, Ta and Hf may be added singly or in combination in a total amount of 0.01 to 1.0%. Preferably, the total amount thereof is 0.03 to 0.6%.
Next, the reasons for the restriction of the incidental impurities causing a serious influence on the propeprties of the steel of the invention is explained below.
Because Ti deteriorates oxidation resistance by forming an internal oxidation phase, the amount of Ti is restricted, as one of the serious impurities, to be not more than 0.1%.
S causes sulfide-base inclusions by reacting with Mn, REM, etc. and lowers the content of rare-earth metals effective in improving the oxidation resistance, with the result that S deteriorates not only the oxidation resistance, but also the hot workability and impairs the steel surface. Accordingly, the amount of S is restricted to be not more than 0.015%. Preferably, the amount of S is not more than 0.008%.
O (oxygen) causes oxide-base inclusions by reacting with Al, Si, Mn, Cr, Y, REM, Zr and etc., and not only deteriorates the hot workability and cold workability, but also reduces the amounts of dissolved Y, REM, Zr, etc., which elements contribute greatly to the improvement in oxidation resistance, thereby reducing the oxidation resistance-improving effect of these elements. Accordingly, the O content is restricted to be not more than 0.010%. Preferably, the O content is not more than 0.008%.
N is an austenite-forming element. Therefore, in a case where N is excessively added to the ferritic Fe-Cr steel of the invention, it makes the austenite phase, thereby not only making it impossible to maintain a single phase of ferrite, but also impairs the hot and cold workability by forming nitride-base inclusions with Al, Cr, etc. Accordingly, the N content is restricted to be not more than 0.050%. Preferably, the N content is not more than 0.020% and more preferably not more than 0.010%.
B not only deteriorates the oxidation resistance by increasing the growth rate of the oxide film at high temperatures of not less than about 700xc2x0 C., but also deteriorates the contact resistance by increasing the surface roughness of the oxide film, which increasing reduces the contact area between the oxide film and the electrode. Accordingly, the amount of B present as one of the serious impurities is restricted to be not more than 0.0030%, and it is preferred to minimize the B content to 0%. A preferred upper limit is not more than 0.0020% and more preferably, the amount of B is less than 0.0010%.
In order to ensure that, in the steel of the invention, Y, REM and Zr which are very effective in improving the oxidation resistance and the electrical conductivity of the oxide film, exert their effect to a sufficient degree, it is necessary that these elements are not completely fixed by the sulfide-base inclusions or oxide-based inclusions. In order to obtain this state, it is effective to suppress the ratios of the amounts of S and O to the contents of Y, Zr and REM into the low level as shown in equation (1).
In a case where the value of equation (1) exceeds 2.0, Y, Zr and REM are fixed by the inclusions, so that these elements do not contribute to the improvement in oxidation resistance and the electrical conductivity of the oxide film. Thus, the value of equation (1) is limited to be not more than 2.0. Incidentally, in making calculations, the value is set to be zero regarding any of the elements of Y, Zr and REM which is not added.
Incidentally, the following elements may be included in the steel of the invention within the following amounts:
Pxe2x89xa60.04%, Cuxe2x89xa60.30%, Mgxe2x89xa60.02%, Caxe2x89xa60.02% and Coxe2x89xa62% 
Next, in fabricating separators of solid-oxide type fuel cells from a steel of any of the above-described chemical composition, it is necessary to form gas passages by cold plastic working and machining. Therefore, it is necessary that the steel be highly capable of cold plastic working and machining and have an impact property high enough to prevent the occurrence of cracks in the steel which are apt to occur due to tensile stresses caused during the heat cycles.
In the invention, hardness, the grain size and impact properties are used as the indices of these required properties.
The hardness, grain size and impact properties, which are the required properties of the steel of the invention for the separators of solid-oxide type fuel cells, are not only determined by alloy compositions alone but also depend greatly on the method of plastic working of the material and the conditions of a heat treatment such as annealing, etc. Therefore, in order that a steel may be used as a steel for the separators of solid-oxide type fuel cells, it is important that not only chemical compositions, but also hardness, grain size, impact properties, etc. meet the ranges limited in the invention.
In order to meet the above-described hardness, grain size and impact properties, it is preferred to perform annealing to remove working strains that occur during the material manufacturing process and that remain in the material. When the annealing temperature is higher than 950xc2x0 C., grains, which are described below, are coarsened. When the annealing temperature is lower than 650xc2x0 C., the softening requires long hours. Furthermore, when the annealing is performed too long at a temperature lower than 650xc2x0 C., the "sgr" sigma phase occurs, so that there occurs such a possibility as the embrittlement is caused. Accordingly, an appropriate annealing temperature range is 650 to 950xc2x0 C.
Incidentally, when the annealing is performed at a high temperature, it is preferred to perform the annealing for a short holding time. On the other hand, at a low temperature it takes long hours to reduce the hardness and, therefore, the holding time can be selected as required from the relationship between the grain size and the hardness.
When the hardness after annealing is higher than 280 HV, the working comes to require a long time and the shape accuracy decreases. Accordingly, the hardness is not more than 280 HV. Preferably, the hardness is not more than 200 HV. When the hardness is within this range, cold working such as cold rolling may be performed after annealing in order to modigy a deformation caused due to thermal strains after annealing.
The steel of the invention for the separators of solid-oxide type fuel cells is used at the use temperatures of not less than about 700xc2x0 C. Because in the fuel cells there are repeated operations and stops, the steel for separators is subjected to a repetition of heat cycles of heating and cooling between the use temperatures and a room temperature. Especially because tensile stresses act during cooling, it is necessary only that the impact properties at room temperature are good in order to prevent the occurrence of cracks during cooling.
In order to obtain good impact properties in a steel of ferrite structure such as the steel of the invention for the separators of solid-oxide type fuel cells, it is effective to reduce the ferrite grain size. In order to obtain the impact properties described below, it is necessary that the average ferrite grain size number be not less than ASTM No. 2, which provides fine grains. Preferably, the average ferrite grain size number is not less than ASTM No. 3, which provides fine grains.
Impact properties can be evaluated by the 2-mm V-notch Charpy impact value at 20xc2x0 C. Impact values of not less than 8 J/cm2 are sufficient, and the impact value is preferably not less than 10 J/cm2.
The steel of the invention for the separators of solid-oxide type fuel cells has excellent electrical resistance particularly in the temperature range of 700 to 950xc2x0 C., and the index of this excellent electrical resistance is defined as explained below.
As an evaluation means of the electrical conductivity, it is important that the electrical conductivity of the oxide film at 750xc2x0 C. after heating 750xc2x0 C. for 1000 hours be not more than 100 mxcexa9xc2x7cm2 and preferably be not more than 50 mxcexa9xc2x7cm2.
Also, as an evaluation means of the phenomenon that the oxidation of the Cr-base oxide film proceeds with the result that the Cr-base oxide film becomes oxide scale and exfoliates, it is important that after heating at 850xc2x0 C. for 100 hours, the exfoliation of the surface oxide scale not substantially occur. Incidentally, xe2x80x9cno exfoliation of surface oxide scale substantially occursxe2x80x9d means that no natural exfoliation of scale occurs in a state where no external impact is applied.
Also, the steel of the invention is a material suitable for the separators of solid-oxide type fuel cells and is often worked into steel sheets and steel strips. However, it is also possible to work this steel in various forms such as steel bar, wire rod, powder, sintered powder material, porous material and steel foil, for use in other parts of solid-oxide fuel cells and parts for other applications in which the characteristics of the steel of the invention can be utilized.